The volume of multimedia traffic (voice, video, image and data) being transmitted across networks, including wireless communication networks, is increasing. To accommodate the increased volume of multimedia traffic, higher throughput, increased reliability, and more efficient use of limited bandwidth is needed. However, wireless communication networks generally have lower bandwidths, harsher time-varying fading characteristics and higher error rates than wired networks. In addition, in some applications, such as military applications, wireless communication networks also need to protect against intentional interference and provide secure transmissions.
Multicarrier Modulation (“MCM”) techniques have been used with wireless communication networks to address these needs. MCM divides a data stream into several parallel streams, each at a much lower bit rate, and then modulates these substreams onto their respective subcarriers (as opposed to the conventional single carrier system). MCM systems include Wavelet Packet Modulation (“WPM”) systems, such as that disclosed in the '834 application. WPM combines multidimensional communications principles and wavelet principles into a multirate wavelet-based modulation format for orthogonally multiplexed communications.
The transmit spectra of the wavelet basis set exhibit significant spectral overlap but are orthogonal. Alias cancellation via power complementary Quadrature Mirror Filter (“QMF”) pairs at the receiver guarantees perfect signal reconstruction on a lossless channel. By relaxing the perfect reconstruction constraint and reorienting the filter tasking in the dyadic trees, a set of orthogonal basis functions with contiguous compact spectral support similar to a channelized filter bank is possible. The waveform's interference avoidance mechanism is thereby extended to be mutually non-interfering with respect to other signals in the vicinity. This is accomplished via selectable spectral gaps bracketing occupied spectral bands. The waveform variant as disclosed herein for non-contiguous spectral operation is known as an Interpolated Tree Orthogonal Multiplexing (“ITOM”) system.
One problem in using WPM or ITOM in a wireless communication network is performing symbol synchronization at the receiver end. Multicarrier modulation systems are particularly sensitive to symbol sampling time offsets because the spectral overlap of the subcarriers can cause significant adjacent channel interference (“ACI”) when timing jitter is present. These systems use orthogonal filtering to divide the baseband data into orthogonal frequency subchannels. This process can be thought of as splitting the spectrum of a Nyquist pulse, resulting in subchannels that retain the Nyquist pulse shape (only the period is affected). The transitions between complex symbols that are modulated using conventional Fourier techniques are captured by edge detection techniques that exploit the shape and polarity of the received pulses to determine the optimal sampling instants. WPM and ITOM produce different (dilated) pulse shapes on each subchannel (also referred to as “subband”) such that the composite, orthogonally multiplexed signal lacks usable transitions. Inspection of the resultant signal constellation (i.e., eye pattern) after WPM or ITOM reveals a nearly continuous footprint (i.e., closed eye). Thus, there is a need for providing symbol synchronization that does not rely on edge detection.
Channel coding has been used to improve the error handling performance of wireless networks. For an even more potent countermeasure to non-Gaussian interference (non-white noise) sources and channel propagation anomalies, the forward error correction (“FEC”) component is distinctly mapped onto the orthogonally multiplexed WPM or ITOM symbols and interleaved to exploit the subband frequency diversity. FEC embodiments may include, but are not limited to, convolutional codes, Reed-Solomon block codes, turbo convolutional and product codes, low density parity check codes, and concatenated code versions (e.g., Reed-Solomon outer code with convolutional inner code).